1. Field
The invention is in the field of hydraulic motors and pumps. More particularly, the invention relates to hydraulic power transfer units which utilize pressure-sensing, fluid flow-limiting and fluid flow bypassing techniques and mechanisms and their operation.
2. State of the Art
Hydraulic power transfer units are commonly used to provide emergency or assistive power from one aircraft hydraulic system to another aircraft hydraulic system. A power transfer unit typically consists of two back-to-back pump/motor units with a common drive shaft, which are connected to different hydraulic systems which may or may not have equivalent working pressures. Each pump/motor unit operates exclusively with hydraulic fluid from the system to which it is connected such that there is no intermixing of hydraulic fluid between systems. Typically, each pump/motor unit contains a hydraulic axial piston pump/motor, or "rotating group" as it is commonly referred to by those skilled in the art.
For convenience, the following discussion is based on systems with equivalent working pressures. If there is a system pressure drop in one system, a differential pressure develops across the power transfer unit and the rotating groups commence rotating or "breakout" to supply power to the lower pressure system. The rotating group which accesses the higher pressure system will act as a motor to rotate the common shaft which drives the rotating group which accesses the lower pressure system as a pump. The power transfer unit will continue to supply power to the lower pressure system until the flow demand to the lower pressure system ceases, the system pressure returns to the rated working pressure and there is no longer an unbalancing differential pressure across the power transfer unit.
Non-bypassing and non-flow-limiting power transfer units are the simplest of power transfer units in that the rotating group which acts as a pump is in direct communication with the hydraulic system with no intermediate valving, such as check or flow limiter valves. Typically, rated or working pressure applied to each side of the power transfer unit will cause the rotating groups to remain in a stalled, or non-rotating state.
When the pressure drops in one system due to a fluid flow demand, the differential pressure between the motor inlet in the high pressure system and the pump outlet in the lower pressure system creates a torque which must exceed the breakout threshold torque of the power transfer unit before the rotating groups will commence rotating. At the breakout threshold, the torque due to differential pressure across the power transfer unit equals the resistive torque due to the static friction of the power transfer unit. Once running, the torque efficiency increases quickly, allowing the rotating groups to accelerate to speed.
With most conventional power transfer units, the high breakout torque characteristics cause the pressure in one system to fall to unacceptably low levels until the power transfer unit breakout threshold is exceeded and the rotating groups begin rotating. Once running, the power transfer unit will provide pressure and fluid flow to the low pressure system until the flow demand falls, and the running torque due to differential pressure across the unit is less than the stall torque threshold, at which point the rotating groups stall and cease rotating. At some low average fluid flow demand rate greater than zero, the power transfer unit will be unable to deliver fluid flow smoothly, and the rotating groups will alternate between stalling and restarting.
One method used in the prior art to avoid the undesirable system pressure fall-off prior to breakout and the undesirable alternative stalling and restarting of the rotating groups with decreasing fluid flow demand and to provide the ability to run smoothly down to zero flow, is the use of internal flow bypassing. Internal flow bypassing avoids power transfer unit stalling under low fluid flow demand by internally diverting a portion of the pump outlet fluid flow into the case or return system, maintaining a fluid flow demand on the rotating group greater than that at which stall occurs. When fluid flow is demanded, the power transfer unit need not overcome the breakout torque since the rotating groups are already rotating. Smooth fluid flow delivery can begin with very little drop in system pressure.
The disadvantage of internal flow bypassing is the consumption of hydraulic power to maintain rotation of the rotating groups at the zero fluid flow condition where no useful work is produced and overall efficiency is zero. This loss is realized as a wasted power draw from the main hydraulic pump, which in aircraft applications is driven by the aircraft engine, and results in undesirable heat rejection into the hydraulic system. When an aircraft is in flight, a power transfer unit typically delivers fluid flow only in the event of an emergency drop in hydraulic pressure or intermittently to provide extra fluid flow during peak system usage. Therefore, unless externally isolated, the rotating groups spend the majority of in-flight operation rotating in the zero fluid flow standby condition due to internal flow bypassing. Over time, the energy and aircraft fuel required for internal flow bypassing can be considerable.
Since the rotating groups of a power transfer unit are not rotationally restrained, the potential for damage to the power transfer unit from overspeed of the rotating groups exists. Overspeed of the rotating groups may occur as a result of a loss of hydraulic load on the pump side of the power transfer unit due to breech-type structural failures in the high-pressure system allowing loss of fluid, or if the power transfer unit drive shaft shears. In addition to possible damage to the power transfer unit, overspeed of the rotating groups could result in a relatively huge consumption of fluid flow by the motor side of the power transfer unit that could exceed the fluid flow capability of the main hydraulic pump, resulting in an unacceptable loss of pressure in that system.
In the prior art, flow limiting valves have been incorporated at the motor inlet side of the power transfer unit to limit the fluid flow consumption of the power transfer unit. The flow limiting valve acts to restrict fluid flow into the motor inlet once the fluid flow has attained a specified value. Fluid flow rate through the flow limiting valve is sensed as a function of differential pressure across an entrance orifice into the flow limiting valve. If the fluid flow increases beyond a specified value, a piston in the flow limiting valve overcomes a spring having a pre-set tension and closes a metering orifice downstream of the entrance orifice. The differential pressure developed across the closing metering orifice lowers the pressure acting across the motor and limits the rotating group speed to an acceptable value.